1. Field of the Invention
The present invention relates to a device tester for detecting abnormality with quiescent power supply current (referred to as IDDQ sometimes hereinafter) by giving a decision on variation in power supply current flowing through a device under test (hereinafter referred to as DUT) such as an integrated circuit and so forth.
2. Description of the Related Art
Following scaleup of integrated circuits and a resultant increase in the number of elements thereof, there has arisen necessity of giving a decision with higher precision on whether or not the integrated circuits perform properly. For this reason, in a function test conducted on, for example, an integrated circuit, it has been a general practice to check up the quiescent power supply current in the integrated circuit as useful means of detecting abnormal leak current and the like occurring thereto caused by failure thereof due to short-circuiting. FIG. 5 is a block diagram showing the configuration of a conventional device tester which has been in widespread use to perform such a test.
In FIG. 5, a DUT 1 denotes a device under test such as the integrated circuit described above and the like. A constant-voltage power supply 2 provides power supply at a predetermined voltage value to the DUT 1 via a detection resistor 3, and such power supply causes a quiescent power supply current to flow through the DUT 1. A current value of the quiescent power supply current is converted into a voltage at a voltage value proportional to the current value by the detection resistor 3. A voltage level adjustment circuit 4 either amplifies or reduces a voltage between opposite ends of the detection resistor 3 such that an output voltage thereof can fall within a range of input voltages suitable for conversion by an A/D converter 5. The A/D converter 5 converts a voltage value delivered from the voltage level adjustment circuit 4 (that is, the voltage value proportional to the current value of the quiescent power supply current) into digital data, and delivers same to a CPU 6 (central processing unit). The CPU 6 gives a decision on whether or not the quiescent power supply current of the DUT 1 is normal on the basis of the digital data delivered from the A/D converter 5 through comparison with a predetermined reference current value, and so forth. In the figure, a capacitor 7 is installed for bypassing noise and so forth.
With the conventional device tester described in the foregoing, however, there are limitations in respect of operational speed and so forth of the A/D converter 5, and in consequence, it takes as long as from several tens of milliseconds to measure one test pattern. It follows therefore that it will take several tens of minutes to give a decision on the quiescent power supply current for all test patterns amounting in number to as many as several ten thousands to several hundred thousands. For this reason, testing of the state of the quiescent power supply current is seldom employed at production lines today except the case of failure or evaluation of integrated circuits. Furthermore, for giving a decision on whether or not the quiescent power supply current is normal, the predetermined reference current value, serving as a reference in giving the decision, needs to be determined beforehand, however, there is a problem that it is difficult to accurately find the value beforehand because the power supply current value varies in actual practice widely depending on process conditions and circuit conditions of individual integrated circuits.